On 09.03.2010 10:22, Mark Nottingham wrote:
> Agreed. Julian, since you did the patch, could you update?
> ...
OK - it appears we want to do multiple things here:
1) clarify input fields (absence of Age header, "local time"), and
2) change the actual calculation (issue 29).
Thus I've decided to do this in two steps. For 1) I just committed
changes <http://trac.tools.ietf.org/wg/httpbis/trac/changeset/778> abd
<http://trac.tools.ietf.org/wg/httpbis/trac/changeset/778>, with the
resulting text being:
-- snip --
2.3.2. Calculating Age
HTTP/1.1 uses the Age response-header to convey the estimated age of
the response message when obtained from a cache. The Age field value
is the cache's estimate of the amount of time since the response was
generated or validated by the origin server. In essence, the Age
value is the sum of the time that the response has been resident in
each of the caches along the path from the origin server, plus the
amount of time it has been in transit along network paths.
The following data is used for the age calculation:
age_value
The term "age_value" denotes the value of the Age header, in a
form appropriate for arithmetic operation; or 0, if not available.
date_value
HTTP/1.1 requires origin servers to send a Date header, if
possible, with every response, giving the time at which the
response was generated (see Section 9.3 of [Part1]). The term
"date_value" denotes the value of the Date header, in a form
appropriate for arithmetic operations. See Section 9.3.1 of
[Part1] for requirements regarding responses without a Date
response header.[[TODO-checkref: correct reference? --jre]]
now
The term "now" means "the current value of the clock at the host
performing the calculation". Hosts that use HTTP, but especially
hosts running origin servers and caches, SHOULD use NTP
([RFC1305]) or some similar protocol to synchronize their clocks
to a globally accurate time standard.
request_time
The current value of the clock at the host at the time the request
resulting in the stored response was made.
response_time
The current value of the clock at the host at the time the
response was received.
A response's age can be calculated in two entirely independent ways:
1. now minus date_value, if the local clock is reasonably well
synchronized to the origin server's clock. If the result is
negative, the result is replaced by zero.
2. age_value, if all of the caches along the response path implement
HTTP/1.1.
These are combined as
corrected_received_age = max(now - date_value, age_value)
When an Age value is received, it MUST be interpreted relative to the
time the request was initiated, not the time that the response was
received.
corrected_initial_age = corrected_received_age
+ (now - request_time)
where "request_time" is the time (according to the local clock) when
the request that elicited this response was sent.
The current_age of a stored response can then be calculated by adding
the amount of time (in seconds) since the stored response was last
validated by the origin server to the corrected_initial_age.
With these definitions, the current_age can be calculated as per:
apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
-- snip --
Before I go on with the actual change for issue 29, I'd like to collect
some feedback on the rearranged text, defining the data for the age
calculation.
Also - Mark - is...:
"See Section 9.3.1 of [Part1] for requirements regarding responses
without a Date response header."
this the reference you had in mind for absent Date headers?
Best regards, Julian